A drive circuit for producing thermal energy by passing a current through a resistor finds use in various equipment and devices. An ink-jet printer is a typical example of such a device. An ink-jet printer of this type has heaters constituted by resistors provided within a printhead and prints an image on a printing medium by utilizing thermal energy, which is produced by passing a current through the heaters, to forcibly eject ink onto the medium from nozzles.
FIG. 11 is a circuit diagram illustrating the general features of a drive circuit for driving a printhead used in an ink-jet printer.
As shown in FIG. 11, one end of each of heaters RHa to RHx constituted by resistors is connected to a first power supply VCC via a power supply wiring resistor rx2. The other ends of the heaters RHa to RHx are connected to the drain terminals of drive transistors M34a to M34x, respectively. The source terminals of the transistors are connected to a power supply GND via a power supply wiring resistor rx3, and drive control signals Fa to Fx are input to the gate terminals of respective ones of these transistors.
By way of example, if the drive control signal Fa attains the H (high) level, transistor M34a turns on and heater RHa becomes a path of current between the power supplies VCC and GND. A current therefore flows into the heater RHa, which produces thermal energy in accordance with the power consumed by the heater. A similar operation will be performed if drive control signals corresponding to the other heaters RHb to RHx, respectively, attain the high level.
The overall printhead has, e.g., 512 heaters corresponding to the nozzles. The heaters are divided into 16 blocks of 32 nozzles each, and the blocks are driven in time-shared fashion as the printhead is moved (i.e., made to scan across the printing medium), thereby achieving a printing scan.
In the circuit for driving the printhead of an ink-jet printer having such a construction, there are cases where, depending upon the image printed, all 32 heaters within the same block are driven simultaneously. In such case, if the ON resistances of the transistors M34a to M34x are sufficiently small and the values of the wiring resistors rx2 to rx3 are made sufficiently small in comparison with the resistance values of the heaters RHa to RHx, then the theoretical value of power Pc consumed by each of the heaters RHa to RHx will be as indicated by the following equation:Pc=(VCC−VEE)2÷RH×k  (1)where k represents a coefficient indicating ON time ratio of the transistors M34a to M34x turned on by the drive control signals Fa to Fx, respectively. By controlling the pulse width of the drive control signals Fa to Fx, therefore, the power consumed by each of the heaters RHa to RHx can be made a desired value.
In order to perform the ink ejection operation accurately in the above-described circuit for driving the printhead of an ink-jet printer, it is necessary to drive the head in such a manner that the amount of heat produced by each heater will exceed a predetermined threshold value.
However, when the amount of heat produced by power consumption in each heater is set so as to sufficiently exceed the amount of heat necessary for ejecting the ink, part of a heater and its surroundings (nozzle) will attain a high temperature if the resistance value of the heater varies. The result is a shorter printhead lifetime.
In order to prevent the above, it is necessary to control properly the power consumed by the heater and to design the circuitry in such a manner that the resistance values of the heaters RHa to RHx will be as uniform as possible.
With the conventional circuitry for driving a printhead, however, the following problem arises: The drive circuit that includes the heater RH is formed on a silicon chip. Consequently, the resistance value of the heater RH exhibits a variation of about ±20% and the consumed power indicated by Equation (1) develops a variation of about 40%. For this reason, the heaters RH are ranked according to range of resistance values and control is performed in such a manner that the input duration (pulse width) of a drive control signal F will take on a length that conforms to each rank. If this approach is adopted, there is not only a decline in printhead productivity but also greater difficulty in terms of control. The overall cost of the apparatus rises as well.
In each rank also there is a variation on the order of ±2% with regard to the resistance value of the heater RH. No measures whatsoever are provided for dealing with this, and some leeway is left when making the settings so that the power consumed by each heater will exceed a predetermined value. This makes it difficult to extend the life of the printhead.
The drive transistors M34a to M34x are designed so as to have a very large gate width in order to reduce the ON resistance. This increases the size of the silicon chip on which the drive circuit is mounted is large and raises cost.
In order to reduce the size of the drive transistors M34a to M34x, a special and costly D (double diffusion) MOS process must be adopted. In this case also, however, control based upon Equation (1) above cannot be applied to the transistors M34a to M34x. The reason for this is that the current driving performance of the MOS transistors M34a to M34x declines with a rise in temperature, i.e., the ON resistance values fluctuate at the operating temperature. As a consequence, some leeway is left when making the settings so that the power consumed by each heater will exceed a predetermined value. This makes it difficult to extend the life of the printhead.
The total value of the currents that flow through the heaters RHa to RHx varies depending upon the image printed and is not constant at all times. In other words, the value of the current that flows through the power supply wiring resistors rx3 and rx4 is not constant and therefore the power consumed by each heater does not take on the value indicated by Equation (1). Consequently, some leeway is left when making the settings so that the power consumed by each heater will exceed a predetermined value. This makes it difficult to extend the life of the printhead.
Furthermore, in order to reduce the values of the power supply wiring resistors rx2, rx3, it is necessary to take into consideration the power supply wiring on the silicon chip on which the drive circuit is mounted and the wiring from the external power supply.
As indicated by Equation (1) above, the power consumed by the heaters is influenced by the power supply voltage. Accordingly, it is necessary to stabilize the value of the external power supply voltage (VCC−VEE). The result is a power supply of higher cost and larger size.